1. Field of the Invention
The present invention relates to an angle detecting device that detects an absolute angle and that has high safety and high reliability.
2. Description of Related Art
An angle detecting device is disclosed in, for example, Japanese Unexamined Patent Application Laid-open No. 2006-250864. This angle detecting device is constructed by combining two of the same type of angle detectors so as to improve redundancy against failure.
The angle detecting device is required to have a function for detecting an absolute angle. The absolute angle is a value of an angle of a rotating shaft of which a rotation angle is a detection target. For example, a resolver with a shaft angle multiplier of 2× outputs signals of two cycles per rotation of a rotating shaft. That is, a data of a rotation angle is obtained at two rotational positions of the rotating shaft (in this case, at angular positions which differ by 180°). Accordingly, an actual rotation angle (that is, an absolute angle) cannot be determined from the data only by the output of the resolver with the shaft angle multiplier of 2×. As in the case just described, in a case in which an angular data obtained from an angle detecting means does not correspond one-to-one with an actual rotation angle, the actual rotation angle of the rotating shaft is called an “absolute angle” in order to differentiate between them.
As an angle detector that detects an absolute angle, a resolver with a shaft angle multiplier of 1× is known. The resolver with the shaft angle multiplier of 1× outputs sine signals and cosine signals of one cycle per rotation, and therefore, an absolute angle can be detected.
In a case of combining the same type of angle detectors, since failure modes thereof are the same, if one of the angle detector has broken down, a probability of the other breaking down is high. Therefore, although the redundancy against failure is improved, the reliability is not high. On the other hand, in a case of using the resolver with the shaft angle multiplier of 1×, an absolute angle can be detected, but detection error is large in principle. Therefore, this resolver is not suitably used for detecting an angle at high accuracy.